IBM Personal Computer with keyboard and monitor
One 8-bit and five 16-bit ISA slots on a motherboard
IBM Personal Computer with keyboard and monitor
8-bit XT, 16-bit ISA, EISA (top to bottom)
Internal view of a PC compatible computer, showing components and layout.
8-bit XT: Adlib FM Sound card
Original IBM Personal Computer motherboard
16-bit ISA: Madge 4/16 Mbps Token Ring NIC
IBM PC with MDA monitor
16-bit ISA: Ethernet 10Base-5/2 NIC
IBM Model F keyboard
8-bit XT: US Robotics 56k Modem
IBM Personal Computer with IBM CGA monitor (model 5153), IBM PC keyboard, IBM 5152 printer and paper stand. (1988)
The back of a PC, showing the five expansion slots
PC DOS 3.30 running on an IBM PC
Digital Research CP/M-86 Version 1.0 for the IBM PC

The bus was (largely) backward compatible with the 8-bit bus of the 8088-based IBM PC, including the IBM PC/XT as well as IBM PC compatibles.

- Industry Standard Architecture

IBM referred to these as "I/O slots," but after the expansion of the PC clone industry they became retroactively known as the ISA bus.

- IBM Personal Computer
IBM Personal Computer with keyboard and monitor

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The Compaq Portable was one of the first nearly 100% IBM-compatible PCs.

IBM PC compatible

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The Compaq Portable was one of the first nearly 100% IBM-compatible PCs.
The original IBM PC (Model 5150) motivated the production of clones during the early 1980s.
The DEC Rainbow 100 runs MS-DOS but is not compatible with the IBM PC.
MS-DOS version 1.12 for Compaq Personal Computers
The PowerPak 286, an IBM PC compatible computer running AutoCAD under MS-DOS.

IBM PC compatible computers are similar to the original IBM PC, XT, and AT that are able to use the same software and expansion cards.

It was later re-named the Industry Standard Architecture (ISA) bus, after the Extended Industry Standard Architecture bus open standard for IBM PC compatibles was announced in September 1988 by a consortium of PC clone vendors, led by Compaq and called the Gang of Nine, as an alternative to IBM's proprietary Micro Channel architecture (MCA) introduced in its PS/2 series.

Compaq

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American information technology company founded in 1982 that developed, sold, and supported computers and related products and services.

American information technology company founded in 1982 that developed, sold, and supported computers and related products and services.

First Compaq logo, used until 1993
Compaq Portable
Compaq Portable 386 BIOS
Aerial map of the Compaq headquarters, now the HP USA campus in unincorporated Harris County, Texas
Former Compaq headquarters, now the Hewlett-Packard United States campus
Post merger logo for Compaq products.
An example of a HP Compaq.

Compaq produced some of the first IBM PC compatible computers, being the second company after Columbia Data Products to legally reverse engineer the IBM Personal Computer.

Although Compaq had become successful by being 100 percent IBM-compatible, it decided to continue with the original AT bus—which it renamed ISA—instead of licensing IBM's MCA.

Motherboard of a NeXTcube computer (1990). The two large integrated circuits below the middle of the image are the DMA controller (l.) and - unusual - an extra dedicated DMA controller (r.) for the magneto-optical disc used instead of a hard disk drive in the first series of this computer model.

Direct memory access

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Feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).

Feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).

Motherboard of a NeXTcube computer (1990). The two large integrated circuits below the middle of the image are the DMA controller (l.) and - unusual - an extra dedicated DMA controller (r.) for the magneto-optical disc used instead of a hard disk drive in the first series of this computer model.
Cache incoherence due to DMA

In the original IBM PC (and the follow-up PC/XT), there was only one Intel 8237 DMA controller capable of providing four DMA channels (numbered 0–3).

With the IBM PC/AT, the enhanced AT Bus (more familiarly retronymed as the ISA, or "Industry Standard Architecture") added a second 8237 DMA controller to provide three additional, and as highlighted by resource clashes with the XT's additional expandability over the original PC, much-needed channels (5–7; channel 4 is used as a cascade to the first 8237).

Intel 8237A-5, used on the original IBM PC motherboard.

Intel 8237

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Direct memory access controller, a part of the MCS 85 microprocessor family.

Direct memory access controller, a part of the MCS 85 microprocessor family.

Intel 8237A-5, used on the original IBM PC motherboard.
Pinout

A single 8237 was used as the DMA controller in the original IBM PC and IBM XT.

For example, the PIIX integrated two 8237 controllers for ISA bus DMA.

IBM System/23 Datamaster

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Announced by IBM in July 1981.

Announced by IBM in July 1981.

The Datamaster was the least expensive IBM computer until the far less expensive and far more popular IBM PC was announced in the following month.

The PC's expansion bus, later known as the ISA bus, was based on the Datamaster's I/O bus.

Closeup of an Intel 8259A IRQ chip from a PC XT.

Intel 8259

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Programmable Interrupt Controller designed for the Intel 8085 and Intel 8086 microprocessors.

Programmable Interrupt Controller designed for the Intel 8085 and Intel 8086 microprocessors.

Closeup of an Intel 8259A IRQ chip from a PC XT.
Pinout
NEC D8259AC, used on the original IBM PC motherboard.

The 8259A was the interrupt controller for the ISA bus in the original IBM PC and IBM PC AT.